Nighttime sheep-herders rejoice, for green-glowing sheep are now a thing. Two questions, though. How? And why?

Photo courtesy Funcación IRAUy/J. Calvelo

Here's the how: last year, a team of researchers led by Alejo Menchaca at the Animal Reproduction Institue of Uruguay genetically modified some wooly ruminants to express a peptide commonly known as green fluorescent protein, or "GFP." GFP glows green when exposed to ultraviolet light, and was originally isolated from the jellyfish Aequorea victoria. Today, it's used to label molecules of biomedical interest – to determine whether those molecules are being expressed, and in what quantity. In the last twenty years, GFP and its derivatives have become such ubiquitous and invaluable research tools that its discoverers were awarded a Nobel Prize back in 2008.

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Menchaca's sheep – which were born last October and, apart from their corporeal glow, appear to have developed just like an unmodified flock – express the protein in tissues throughout their bodies, causing them to glow under ultraviolet light. Menchaca says that, in this particular instance, the sheep were modified merely as a proof of concept. As in: Look everybody, we can breed a totally healthy sheep that also happens to glow. Isn't that swell? Well yes, it is. It's kind of eerie (look into the eyes of the GFP-sheep up top and tell me you aren't afraid), but also pretty cool. But why make a sheep glow in the first place?

GFP animals are a growing (glowing?) trend in biomedical research. To date, scientists have genetically engineered a wide swath of the animal kindgdom to express GFP, a recent example being this litter of glow-in-the-dark kittens. Because GFP can be engineered to show up only when it's expressed with another, specific protein, researchers can use glowing animals to study and better understand biological processes that occur on the whole-body scale. In the case of the cats, the green glow is used to help researchers identify, at a glance, the body-wide expression of an antiviral protein that could prove useful in suppressing the spread of HIV.

One of Menchaca's research interestes is the development of a genetically modified sheep that can produce milk imbued with human growth hormones – milk which could be used to help treat humans suffering from endocrine disorders. It's not hard to imagine how his team might use GFP to clearly label which sheep have been born with the ability to produce these hormones. This, of course, is just one potential application of the technology, which Menchaca says is still very much in development.

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“The technique is complex and demands much work and is one of the limiting factors, so despite the global interest and demand it is still a slow process," he said in a statement.

"Our focus is generating knowledge and making it public so the scientific community can be informed and help in the long run march to generate tools so humans can live better."